1. Field of the Invention
The invention relates to field emitters, and more particularly to lateral luminescent field emitter devices in flat panel displays.
2. Description of Related Art
There are several means by which electrons may be emitted from a material by increasing the energy of electrons at the material surface so that the energy exceeds a certain energy potential barrier. For example, thermionic emission uses heat, photoemission uses radiation such as light, and secondary emission uses charged particles such as electrons or ions to increase the electron energy level at the emission surface. Electron emission by such means has been used for cathode ray tubes in television sets for example.
Field emission devices ("FED's") liberate electrons by lowering the potential barrier at a conductive emission surface rather than by raising the electron energy. In accordance with the probabilities of quantum mechanics, although the energy of electrons in a conductive material does not exceed the potential barrier at the conductor surface nevertheless a certain portion of those electrons will tunnel through that potential barrier to be emitted at the surface. An electrical field may be employed to narrow the potential barrier so that an increasing portion of the electrons are emitted, thereby increasing field emission current. Such FED's have been used for purposes such as electron microscopes and flat panel displays. They have been extensively studied and are well known in the art. See, for example, R. J. Noer, "Electron Field Emission from Broad Area Electrodes", Applied Physics A 28, pp. 1-24 (1982).
FED's have a number of limitations which restrict their usefulness. One limitation concerns the energy level imparted to the electrons after they are emitted. Another limitation concerns the uniformity of emission current. The mechanisms and tradeoffs of these and other limitations will be further explained in the following discussion.
One limitation concerns ionization due to electron energy. The energy which the electric field imparts to electrons after emission may reach a level that causes gases surrounding the electron emission surface to ionize. Such ionized gases may in turn damage the emission surface and impair further emission. See, for example, U.S. Pat. No. 3,970,887, by D. Smith, et al., entitled "Micro-Structure Field Emission Electron Source" (discussing shortened life due to ionization). Therefore, to reduce the required electrical field and thereby reduce the amount of ionization, typical FED's use low "work function" materials for the emission surface, that is, special materials that emit electrons at relatively low energy levels. R. Gomer, FIELD EMISSION AND FIELD IONIZATION, Harvard Univ. Press, pp. 3-4 (1961); see also, U.S. Pat. No. 4,663,559, by A. Christensen, entitled "Field Emission Device".
The electrical field required for emission may also be reduced by shaping the emission surface so that the field is concentrated into a small region. See, for example, U.S. Pat. No. 3,998,678, by S. Fukase, et al., entitled "Method of Manufacturing Thin-Film Field Emission Electron Source" (conical tips); U.S. Pat. No. 4,663,559, by A. Christensen, entitled "Field Emission Device" ("whiskers" in prior art and particles in the Christensen device); and U.S. Pat. No. 5,066,883, by S. Yoshioka, et al., entitled "Electron-Emitting Device with Electron-Emitting Region Insulated from Electrodes" (thin film with cracks); and U.S. Pat. No. 5,089,742, by D. Kirkpatrick, et al., entitled "Electron Beam Source Formed with Biologically Derived Tubule Materials" (micro-protrusions); V. Makhov, "Field Emission Cathode Technology and its Application", Technology Digest of IVMC 91, Nagahama 1991 (edge of film). This results in emission at an applied voltage lower than the voltage required for a reference configuration with flat shapes, thereby defining a "field enhancement factor". See H. Busta, et al., "Field Emission from Tungsten-Clad Silicon Pyramids", IEEE Transactions on Electron Devices, Vol. 36, No. 11, pg. 2679 (November 1989).
One drawback of concentrating the field in a small region is that the current emission is also limited to a small region resulting in a low current, high density electron beam. See U.S. Pat. No. 3,755,704, by C. Spindt, et al., entitled "Field Emission Cathode Structures and Devices Utilizing Such Structures" (discussing techniques to provide multiple points for parallel currents in order to increase total current available). And the typical sharp pointed emitters also suffer from uniformity limitations. See H. Kosmahl, "A Wide-Bandwidth High-Gain Small-Size Distributed Amplifier with Field-Emission Triodes (FETRODE's) for the 10 to 300 GHz Frequency Range", IEEE Transactions on Electron Devices, Vol. 36, No. 11, pg. 2728 (November 1989) (explaining that sharp pointed structures do not provide uniform emission currents from one device to the next, and discussing how the variation relates to topography). Thus, such beams are not ideally suited for producing luminesence over a large area.
Besides using low work function material and field enhancing shapes to reduce the required electrical field for electron emission, FED's typically employ a small separation between the emission electrode and the accelerator electrode (i.e., "field electrode", or "gate") which produces the liberating electrical field. In this manner the electrical field is increased without increasing the voltage driving the field so that less energy is imparted to the electrons after emission. One means for providing such a small separation involves etching a laminate structure with a first electrode on a flat substrate, a thin dielectric layer over the first electrode, and a second electrode layer over the dielectric so that the bottom electrode is exposed in close proximity to the top electrode. See, for example, U.S. Pat. No. 4,307,507, by H. Gray, et al., entitled "Method of Manufacturing a Field-Emission Cathode Structure"; U.S. Pat. No. 4,943,343, by Z. Bardai, et al., entitled "Self- Aligned Gate Process for Fabricating Field Emitter Arrays"; and U.S. Pat. No. 4,964,946, by H. Gray, et al., entitled "Process for Fabricating Self-Aligned Field Emitter Arrays"; U.S. Pat. No. 5,066,883, by S. Yoshioka, et al., entitled "Electron-Emitting Device with Electron-Emitting Region Insulated from Electrodes". Another means for providing a small separation between electrodes involves etching a buffer layer between electrodes on the same substrate to provide lateral electron emission. See, for example, Makhov, "Field Emission Cathode Technology and its Application", Technical Digest of IVMC 91, Nagahama 1991; S. Bandy, "Thin Film Emitter Development", Technical Digest of IVMC 91, Nagahama 1991.
There are tradeoffs involved in lowering the voltage required to produce electron emission. It is desirable to reduce the voltage not only in order to reduce gas ionization, but also because it increases frequency response by reducing the time required to bring the field electrode up to the required emission voltage. As described above, reducing the separation between the emission electrode and the field electrode helps to reduce the required voltage; however, a decreasing separation has the undesireable side effect of increasing sensitivity of the FED emission current to small variations in electrode separation. FED current density may change by as much as 10% for a 1% change in electrode separation. Furthermore, although it is desirable to lower the energy level imparted to emitted electrons in order to preserve the emission surface, it is also desirable to impart a relatively high energy level to the electrons so that they may deliver more energy to generate more light in a display for example. Higher energy is especially needed where the total emission current is limited by current density when field enhancing shapes are employed. See U.S. Pat. No. 3,665,241, by C. A. Spindt, et al., entitled "Field Ionizer and Field Emission Cathode Structures and Methods of Production" (discussing low current because of the minute size of a sharp pointed emitting area, and low energy because of the small separation between emitter and accelerator electrodes). Thus to raise the allowable operating voltage and limit ionization damage to emission surfaces a high vacuum is typically employed. See, for example, U.S. Pat. No. 4,663,559, by A. Christensen, entitled "Field Emission Device" (discussing typical vacuum operation).
To accommodate these tradeoffs FED's have typically been triode arrangements wherein a high voltage anode is employed above a field (i.e., "accelerator") electrode. In these devices a low voltage field electrode is placed on the same substrate in a layer above the emission surface electrode. A small separation between the emitter ("cathode") and field electrodes may thus be precisely controlled so that emission occurs uniformly and at a low voltage. A higher voltage electrode ("anode") is then provided on another substrate aligned above the first. See, for example, U.S. Pat. No. 5,066,883, by S. Yoshioka, et al., entitled "Electron-Emitting Device with Electron-Emitting Region Insulated from Electrodes" (for example FIG. 3B indicating electron emission 7 toward a third electrode not shown); U.S. Pat. No. 4,780,684, by H. Kosmahl, entitled "Microwave Integrated Distributed Amplifier with Field Emission Triodes" (conical or pyramid shaped emitters in triode structure); and H. Kosmahl, "A Wide-Bandwidth High-Gain Small-Size Distributed Amplifier with Field-Emission Triodes (FETRODE's) for the 10 to 300 GHz Frequency Range", IEEE Transactions on Electron Devices, Vol. 36, No. 11, pg. 2728 (November 1989).
Although emission current uniformity is increased by providing a low voltage field electrode precisely located nearby the cathode, nevertheless, in these triode FED's spacing between the anode and cathode is still very important. Variations in anode-cathode spacing may cause image distortion and non-uniform brightness in flat panel displays. Spacers used to secure the anode-cathode separation may permit leakage current which increases power consumption, distorts the electrical field, and contributes to electrode breakdown. And since a vacuum of less than 10.sup.-6 torr is generally applied between the anode and cathode substrates the spacers must be strong and numerous to withstand the forces on the substrates. Thus precise spacing is problematic for the triode FED's used in flat panel displays where the anode and cathode are on two different substrates. See, for example, U.S. Pat. No. 4,923,421, by I. Brodie, et al., entitled "Method for Providing Polyimide Spacers in a Field Emission Panel Display".
Thus the above structures and atmospheres are useful for flat panel display applications, but they may be further improved. The field enhancing structures such as conical tips, film edges, whiskers, etc. provide only a low current, high density electron beam. Such beams are not ideally suited for producing luminesence over a large area. Also these structures suffer from uniformity limitations. Triodes are not ideally suited to flat panel displays because they require precise alignment in two planes and the spacers are problematic. Furthermore, none of these structures provide a broad area emission surface to direct electrons laterally to a luminescent anode located, as is desirable for a flat panel display.